Polypropylene occurs in crystal states such as α-crystals and β-crystals, and β-crystals can be produced preferentially by employing special crystallization conditions or by adding a β-crystal nucleating agent. β-Crystals are known to undergo a transition into stable α-crystals when subjected to thermal and dynamic action, and recently several methods have been proposed for producing an air-permeable polypropylene film having continuous through-pores, which make use of the crystal transition that occurs in the course of stretching (Japanese Unexamined Patent Publications H7-118429, H9-176352, H9-255804, and H6-100720). However, the pore formation mechanism involving β-crystals is complicated, and is not yet fully understood. Consequently, these methods have failed to produce a porous film in a stable manner.
In order to obtain a porous film, all of the above publications recommends to form β-crystals in a largest possible amount in the unstretched web sheet before stretching, and then carry out stretching at an optimal temperature, wherein a K value determined by X-ray diffraction is employed as an index of the β-crystal content. A K value of 1.0 indicates a β-crystal content of 100%, and with the understanding that the higher the K value is, the easier it is to obtain a porous film with high air-permeability, it is recommended in Japanese Unexamined Patent Publication H9-255804, for instance, that the K value be at least 0.7, preferably 0.8 to 0.98. The recommended stretching temperature is about 50 to 100° C. during longitudinal stretching, and about 100 to 150° C. during transverse stretching.
These recommended K values are achieved relatively easily by adding a specific β-crystal nucleating agent, without having to employ special crystallization conditions. However, stretching a sheet having a high K value at the recommended temperature does not necessarily give a porous film with high permeability.
For example, the strain rate during stretching affects pore formation, and there is a strong tendency for pore formation to be impaired if the strain rate is high during transverse stretching in particular. No pores may sometimes be formed, if stretching is carried out at a transverse stretching strain rate of at least 60 times/ minute (or 100%/second) which is usually used in the manufacture of an ordinary nonporous, biaxially stretched polypropylene film. The strain rate is determined as the ratio V/D (or 100V %/D) of the stretching rate V to the sample dimension D in the stretching direction, and an extremely slow strain rate of less than 10 times/minute (17%/second) (longitudinal and transverse directions) is recommended as a condition for forming pores in Japanese Unexamined Patent Publication H6-100720. However, decreasing the strain rate is undesirable because it leads to lower productivity.
There are also cases in which no pores are formed even if the K value is high and preferable stretching temperature and slow strain rate are employed. The mechanism by which pores are formed through β-crystals is complicated, and stable industrial manufacturing conditions had to be established, with plenty of room remaining for improvement.
Aside from the problem of pore formation, another serious problem up to now has been the breakage of a film during its manufacture. This breakage is ameliorated by utilizing β-crystals, as compared to a porous film containing a filler such as calcium carbonate, but the results are still not satisfactory, and further improvement is needed.
In recent years, porous polypropylene films have been used in a wide variety of fields depending on the characteristics thereof. Specifically, they found use in disposable diapers, feminine sanitary products and packaging materials due to their water vapor permeability, in synthetic paper and wallpaper materials due to their printing characteristics, and in filtration membrane and battery separators due to their separation characteristics.
Their use as battery separators has been especially popular because the demand for electronic devices has recently soared. Examples of the application of a porous polypropylene film containing β-crystals have been proposed in several places, such as Japanese Unexamined Patent Publication 2000-30683.
One of the most important properties of a battery separator is its electrical resistance. Electrical resistance is the measured value of the resistance to current flowing through a separator between a cathode and an anode, and is known to be proportional to the product of the Gurley air-permeability and the pore size (“Kagaku Kogyo ” [Chemical Industry], January issue (1997) or R. W. Callahan et al., The Tenth International Seminar on Primary and Secondary Battery Technology and Application, Mar. 1–4, 1993). It is generally preferable that this electrical resistance be as low as possible, and in more specific terms, the electrical resistance per mil (25 μm) of thickness is preferably less than 30 ohm·in, more preferably less than 20 ohm·in.
Japanese Unexamined Patent Publication 2000-30683 discloses several recommended stretching conditions, including the temperature and stretch ratio during longitudinal and transverse stretching, and the total stretching range. Nevertheless, a battery separator comprising the porous film disclosed in Japanese Unexamined Patent Publication 2000-30683 is not necessarily satisfactory in terms of thickness uniformity, and even if the recommended stretching conditions given in this publication are employed, the resulting porous film did not necessarily have the electrical resistance required of a battery separator.
Therefore, there has been a need to better understand the pore formation mechanism, and to establish an industrially optimal manufacturing method that is suited to this mechanism. In particular, the thickness uniformity of a porous film was inadequate in the past, and consequently there was unsatisfactory uniformity in film characteristics, such as air-permeability, tensile characteristics, electrical resistance and porosity, and there was variance in the manufactured film from place to place. Therefore, there is a need for the development of a porous film with superior thickness uniformity, as well as a process for producing such a film.
It is an object of the present invention to solve these problems, and in particular to provide a porous polypropylene film which has good thickness uniformity and high porosity and air-permeability, and which preferably has the electrical resistance required of a battery separator.
It is a further object of the present invention to provide a manufacturing process for preparing a porous polypropylene film which is resistant to breakage during manufacture, by which the film can be manufactured in a stable manner and at a high strain rate.